Police also were investigating a report of an incident on a bus in east London.

London Underground said there were no immediate reports of casualties. One witness said he had been told by another subway passenger that a rucksack had exploded on a train.

Police said Warren Street, Shepherds Bush and Oval underground stations had been evacuated. Emergency services personnel were called to the stations, police said.

At Warren Street there were reports of a nail bomb explosion. Sky's Crime Correspondent Martin Brunt said police believe this may have been the sound of detonators going off.

"People were panicking. But very fortunately the train was only 15 seconds from the station," witness Ivan McCracken told Sky news.

McCracken said another passenger had claimed he had seen a rucksack explode. The bombs which killed 56 people on board three underground trains and a bus in London on July 7 were carried in rucksacks, police said.

McCracken said he smelled smoke and that people were panicking and coming into his carriage.

"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack," McCracken said.

"The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."

Services on the Victoria and Northern lines were suspended following reports of a number of incidents, London Underground said.